1. Field
Thermal interface materials.
2. Description of Related Art
In order to meet the market demand for high performance microprocessors, the recent trend in microprocessor architecture has been to increase the number of transistors (higher power), shrink processor size (smaller die), and increase clock speeds (higher frequency). These trends have resulted in the escalation of power dissipation as well as heat flux at the silicon die level, which increase both the raw power as well as power density on silicon.
Thermal materials have been used in packaging as interfaces between devices to dissipate heat from these devices (e.g., microprocessors). One typical thermal interface material (TIM) typically includes a polymer matrix and a thermally conductive filler. The TIM technologies used for electronic packages encompass several classes of materials such as phase change materials, epoxies, greases, and gels.
Phase change materials (PCMs) are in a class of materials that undergo a transition from a solid to a liquid phase with the application of heat. These materials are in a solid state at room temperature and are in a liquid state at die operating temperatures. When in the liquid state, PCMs readily conform to surfaces and provide low thermal interfacial resistance. PCMs offer ease of handling and processing due to their availability in a film form and the lack of post dispense processing. However, from a formulation point, the polymer and filler combinations that have been utilized in PCMs restrict the bulk thermal conductivities of these materials.
Metal filled epoxies commonly are highly conductive materials that thermally cure into highly crosslinked materials. They, however, have significant integration issues with other components of the package. For example, metal filled epoxies exhibit localized phase separation within the material. This is driven by package thermo-mechanical behavior that results in high contact resistance. Furthermore, the high modulus nature of epoxies leads to severe delamination at the interfaces.
Thermal greases offer several advantages compared to other classes of materials including good wetting and ability to conform to the interfaces, no post-dispense processing, and high bulk thermal conductivity. Greases provide excellent performance in a variety of packages; however, greases cannot be used universally with all packages due to degradation of thermal performance during temperature cycling. It is observed that in some packages greases migrate out from between the interfaces under cyclical stresses encountered during temperature cycling. This phenomenon is known as “pump out.” The extensive thermo-mechanical stresses exerted at the interface during temperature cycling are due to the relative flexing of the die and the thermal plate with changes in temperature. Because the pump-out phenomenon is inherently related to the formulation chemistries utilized in greases, all typical greases are subject to pump-out.
Many high performance, high power processors require the use of integrated heat spreaders (IHSs). In general, the well-known thermal greases, epoxies, and phase change materials that are currently available in the market do not meet the performance requirement for packages comprising an IHS. PCMs do not possess high enough bulk thermal conductivities necessary to dissipate the high heats from the central processing units, and they typically require the use of external clamps for the application of constant positive force for optimum performance. The highly conductive metal filled epoxy thermal polymers can not be used due to integration issues that lead to delamination and high interfacial resistance. And, greases are limited due to pump-out.
Gels typically include a crosslinkable silicone polymer, such as vinyl-terminated silicone polymer, a crosslinker, and a thermally conductive filler. Gels combine the properties of both greases and crosslinked TIMs. Before cure, these materials have properties similar to grease. They have high bulk thermal conductivities, have low surface energies, and conform well to surface irregularities upon dispense and assembly, which contributes to thermal contact resistance minimization. After cure, gels are crosslinked filled polymers, and the crosslinking reaction provides cohesive strength to circumvent the pump-out issues exhibited by greases during temperature cycling. Their modulus (E′) is low enough (in the order of mega-pascal (MPa) range compared to giga-pascal (GPa) range observed for epoxies) that the material can still dissipate internal stresses and prevent interfacial delamination. Thus, the low modulus properties of these filled gels are attractive from a material integration standpoint.
Even though the modulus of the gel-type TIMs currently used in the industry is low, it is not low enough to survive the reliability-stressing test. Commonly assigned U.S. Pat. No. 6,469,379 describes a curable TIM that has lower modulus and improved performance requirements of electronic packages and also survives reliability-stressing tests. In one embodiment, U.S. Pat. No. 6,469,379 describes a TIM including, a vinyl terminated silicon polymer; a silicone cross-linker having terminal silicon-hydride units; a chain extender; and a thermally conductive filler such as a metal (e.g., aluminum, silver, etc.) and/or a ceramic (e.g., aluminum nitride, aluminum oxide, zinc oxide, etc.). While this material has a lower modulus and improved performance over other TIMs, the thermal performance of the material may still not be suitable for performance requirements of current and future generation electronic products.